1. Field of the Invention
The present invention relates to an electron injection composition for a light-emitting element, a light-emitting element formed with the use of the electron injection composition, and a light-emitting device that has the light-emitting element.
2. Description of the Related Art
A light-emitting element that uses a material as a light emitter, which has features such as a thin thickness and lightweight, a high speed response, and low DC voltage drive, has been expected to be applied to a next-generation flat panel display. In addition, it is said that a light-emitting device that has light-emitting elements arranged in a matrix shape is superior in having a wide view angle and a high level of visibility, as compared to a conventional liquid crystal display device.
A light-emitting element is said to have an emission mechanism that: an electron injected from a cathode and a hole injected from an anode are recombined in the luminescence center of a layer including luminescent material to form an excited molecule when a voltage is applied with the layer including the luminescent material between the pair of electrodes; and energy is released to emit light while the excited molecule moves back toward a ground state. As the excited state, a singlet excited state and a triplet excited state are known, and luminescence is said to be possible through any of the singlet excited state and the triplet excited state.
As for such a light-emitting element as this, there are a lot of problems depending on materials against improving characteristics of the element, and therefore, in order to overcome the problems, the structure of the element has been improved and materials for the element has been developed, for example.
One of the problems depending on materials is that there are few appropriate materials for forming a transparent conductive film. As an appropriate material for forming a transparent conductive film, a material that has a large work function (specifically a work function of 4.0 eV or more) such as ITO (indium tin oxide) or IZO (indium zinc oxide) of indium oxide mixed with zinc oxide (ZnO) at 2 to 20% is known. Since the transparent conductive film as mentioned above is used for a transparent electrode to take light from a light-emitting element to the outside, it is usually the case that the transparent electrode functions as anode of the light-emitting element.
On the contrary, it is reported that an electron injection from a transparent electrode can be improved to make the transparent electrode formed of a material that has a large work function such as ITO function as a cathode when a layer for improving an electron injection from an electrode (hereinafter, referred to as an electron injection layer) is formed in contact with the transparent electrode (refer to Non-Patent Documents 1 and 2, for example). In this case, an electron transport material (for example, tris(8-quinolinolato)aluminum (hereinafter, referred to as Alq3), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also referred to as bathocuproin and hereinafter, referred to as BCP), and copper phthalocyanine (hereinafter, referred to as Cu-Pc)) is doped with an alkali metal to form the electron injection layer.
In addition, it is also reported that an electron injection from an electrode can be remarkably improved in the case where BCP among electron injection layers, which is known as a material that is superior in transporting only electron, is doped with Li.
However, it is difficult to keep amorphous when BCP is used to form a film, and BCP also has a defect of being easy to crystallize with time. Therefore, in the case of forming an element, deterioration in device characteristics such as fluctuations in luminance is caused due to a change in luminous efficiency, and the element also has a problem of a shortened lifetime due to deterioration in luminance.
(Non-Patent Document 1)
Junji Kido, Toshio Matsumoto, Applied Physics Letters, Vol. 73, No. 20 (16 Nov. 1998), 2866-2868
(Non-Patent Document 2)
G. Parthasarathy, C. Adachi, P. E. Burrows, S. R. Forrest, Applied Physics Letters, Vol. 76, No. 15 (10 Apr. 2000), 2128-2130